1. Field of the Invention
The present invention relates to a chemical vapor deposition process. More particularly, the present invention relates to a method that improves the inconsistency of the wafer thin film thickness by chemical vapor deposition.
2. Description of the Related Art
Chemical vapor deposition is defined as the formation of a nonvolatile solid film on a substrate by the reaction of the vapor phase reactants that contain the desired components. If the reactant is a liquid at room temperature and atmospheric pressure, the reactant is vaporized before it is introduced into the reaction chamber.
There are two types of design for the reactant supply unit of a chemical vapor deposition system or reactor for the reactant, which is a liquid at room temperature. The first design heats the liquid reactant to increase the vapor pressure of the liquid reactant. The second design delivers a fixed amount of the carrier gas into the container of the liquid reactant. Using the carrier gas, the partial vapor pressure of the liquid reactant increases the flow rate of the liquid reactant is thus increased. The latter design has been more widely used because it is more convenient to control.
FIG. 1 is a schematic diagram showing parts of a chemical vapor deposition system using a carrier gas to deliver the reactant in a Very Large Scale Integrated circuit (VLSI) processing. As shown in FIG. 1, the liquid reactant, for example, tetrakisdimethyl-amidotitanium (TDMAT) is contained in an ampule 10 (or a bubbler chamber). The carrier gas, for example, helium (He), is delivered in a fixed flow rate into the ampule 10 by a mass flow controller 12 (MFC) to carry the TDMA vapor into a reaction chamber 14 for a chemical vapor deposition reaction after passing through the piping and combining with gases required for other processing conditions.
Using the mass flow controller 12 (MFC) to control the flow rate of the carrier gas delivered to the TDMAT, the flow rate of TDMAT is also controlled. By presetting the processing recipe of a particular process, the deposition system or reactor is automated to perform the chemical vapor deposition process.
After the reactor has been functioning for a certain time period, the thickness of the thin film deposited on the wafer, however, is different from the preset value without altering the pre-set condition. In another words, the flow rate of the carrier gas is supposed to remain unchanged. The extent of the difference between the preset value and the actual thickness of the deposited thin film increases as the number of times of the reactor is used increases.
These results indicate that the amount of the reactant being delivered into the reaction chamber 14 alters as the number of times and the duration of the operation accumulate. The resulting film thickness thus deviates from the preset value, which would lead to an inconsistent thin film thickness deposited on the different batches of the wafer. The quality of the product is therefore out of control.
Furthermore, there are at least two reaction chambers for each chemical vapor deposition system and each reaction chamber has its individual ampule 10. Since the number of operations accumulated for each reaction chamber is different, the extent of the deviation of the thickness of the deposited thin film from the preset value is different for each reaction chamber. In another words, the consumption rate of the reactant is different for different reaction chambers. The consumption rate of the reactant in each ampule, therefore, can not be exactly determined to provide a dependable schedule for replacing the ampule.